The need for optical fiber waveguide connectors in optical fiber waveguide communication systems and other applications has long been apparent. Ideally, connectors should present only a minimal loss in the fiber waveguide transmission medium. There are two basic types of connectors: the fiber waveguide to fiber waveguide connector, and the collimated beam free-space connector. Fiber-to-fiber waveguide connectors are the simplest and least expensive; however, they are extremely sensitive to misalignment. To keep losses below a tenth of a decibel (0.1 dB) in a typical single mode fiber-to-fiber waveguide connector, the gap between the fibers and any lateral misalignment must be kept below two microns. Most fiber-to-fiber waveguide connectors in the past have depended on axial alignment of the components in the connector to minimize loss. Typical of these are ferrule-type connectors, which produce the best tolerances. Typical losses for the best of these connectors, single mode, are 0.2 dB mean and 0.3 dB at three standard deviations. A significant problem with these connectors is the contact. Since it is virtually impossible to maintain spacing on the order of one micron, the common practice is to butt the fiber waveguides to one another. However, since such connectors are often deployed in a high vibration environment, such as an aircraft, they tend to degrade over time as a result of vibration-induced spalling of the ends of the fibers.
One of the solutions to this issue is the free-space collimated connector. Although any lens can be used in such connectors, ball and GRIN lenses are most often used to form a collimated beam. Collimating a beam amounts to trading spatial sensitivity for angular sensitivity. For example, to maintain a 0.2 dB insertion loss, the optical axis of a two millimeter focal length lens must be aligned to within three minutes of arc for a single-mode fiber waveguide.
The majority of collimated lens connectors align the fiber waveguide to the outside diameter of the lens and the inside diameter of the ferrule. In other words, the outside radii of the fiber waveguide is referenced to the outside radii of the lens; this radially-referenced axis is then used to align the halves together. Unfortunately, this alignment technique requires great precision, otherwise significant losses can result. The most important aspect is controlling the tolerances on the GRIN lens, including both the diameter and angle of its face. Another aspect is aligning the mechanical center of the fiber waveguide precisely with the mechanical center of the lens. Failure to do so can result in significant insertion loss 0.75 to 1.5 dB and variation in the insertion loss.